Bacterial cells, such as E. coli, are commonly used for producing recombinant proteins. There are many advantages to using bacterial cells, such as E. coli, for producing recombinant proteins particularly due to the versatile nature of bacterial cells as host cells allowing the gene insertion via plasmids. E. coli have been used to produce many recombinant proteins including human insulin.
Despite the many advantages to using bacterial cells to produce recombinant proteins, there are still significant limitations including the difficulty of producing protease sensitive proteins. Proteases play an important role in turning over old, damaged or misfolded proteins in the E. coli periplasm and cytoplasm. Bacterial proteases act to degrade the recombinant protein of interest, thereby often significantly reducing the yield of active protein.
A number of bacterial proteases have been identified. In E. coli proteases including Protease III (ptr), DegP, OmpT, Tsp, prlC, ptrA, ptrB, pepA-T, tsh, espc, eatA, clpP and lon have been identified.
Tsp (also known as Prc) is a 60 kDa periplasmic protease. The first known substrate of Tsp was Penicillin-binding protein-3 (PBP3) (Determination of the cleavage site involved in C-terminal processing of penicillin-binding protein 3 of Escherichia coli; Nagasawa H, Sakagami Y, Suzuki A, Suzuki H, Hara H, Hirota Y., J Bacteriol. 1989 November; 171(11):5890-3 and Cloning, mapping and characterization of the Escherichia coli Tsp gene which is involved in C-terminal processing of penicillin-binding protein 3; Hara H, Yamamoto Y, Higashitani A, Suzuki H, Nishimura Y., J Bacteriol. 1991 August; 173 (15):4799-813) but it was later discovered that the Tsp was also able to cleave phage tail proteins and, therefore, it was renamed as Tail Specific Protease (Tsp) (Silber et al., Proc. Natl. Acad. Sci. USA, 89: 295-299 (1992)). Silber et al. (Deletion of the prc(tsp) gene provides evidence for additional tail-specific proteolytic activity in Escherichia coli K-12; Silber, K. R., Sauer, R. T.; Mol Gen Genet 1994 242:237-240) describes a prc deletion strain (KS 1000) wherein the mutation was created by replacing a segment of the prc gene with a fragment comprising a Kanr marker.
The reduction of Tsp (prc) activity is desirable to reduce the proteolysis of proteins of interest. However, it was found that cells lacking protease prc show thermosensitive growth at low osmolarity. Hara et al isolated thermoresistant revertants containing extragenic suppressor (spr) mutations (Hara et al., Microbial Drug Resistance, 2: 63-72 (1996)). Spr is an 18 kDa membrane bound periplasmic protease and the substrates of spr are Tsp and peptidoglycans in the outer membrane involved in cell wall hydrolysis during cell division. The spr gene is designated as UniProtKB/Swiss-Prot P0AFV4 (SPR_ECOLI).
Improved protease deficient strains comprising mutant spr genes have been described. Chen et al describes the construction of E. coli strains carrying different combinations of mutations in prc (Tsp) and another protease, DegP, created by amplifying the upstream and downstream regions of the gene and ligating these together on a vector comprising selection markers and a sprW174R mutation (High-level accumulation of a recombinant antibody fragment in the periplasm of Escherichia coli requires a triple-mutant (ΔDegP Δprc sprW174R) host strain (Chen C, Snedecor B, Nishihara J C, Joly J C, McFarland N, Andersen D C, Battersby J E, Champion K M. Biotechnol Bioeng. 2004 Mar. 5; 85(5):463-74). The combination of the ΔDegP, Δprc and sprW174R mutations were found to provide the highest levels of antibody light chain, antibody heavy chain and F(ab′)2-LZ. EP1341899 discloses an E. coli strain that is deficient in chromosomal DegP and prc encoding proteases DegP and Prc, respectively, and harbors a mutant spr gene that encodes a protein that suppresses growth phenotypes exhibited by strains harboring prc mutants.
The present invention provides new bacterial strains carrying alternative spr mutants which provide advantageous means for producing recombinant proteins.